LM5021MM-1/NOPB National Semiconductor, LM5021MM-1/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM5021MM-1/NOPB
• Description: IC PWM CTRLR AC-DC CURR 8-MSOP
• Manufacturer: National Semiconductor

Specifications of LM5021MM-1/NOPB

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 85%
• Voltage - Supply: 8 V ~ 30 V
• Buck: Yes
• Boost: Yes
• Flyback: Yes
• Inverting: Yes
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Frequency-max: 1MHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM5021MM-1; LM5021MM-1TR

SKIP CYCLE OPERATION
During light load conditions, the efficiency of the switching
power supply typically drops as the losses associated with
switching and operating bias currents of the converter be-
come a significant percentage of the power delivered to the
load. The largest component of the power loss is the switching
loss associated with the gate driver and external MOSFET
gate charge. Each PWM cycle consumes a finite amout of
energy as the MOSFET is turned on and then turned off.
These switching losses are proportional to the frequency of
operation. The Skip Cycle function integrated within the
LM5021 controller reduces the average switching frequency
to reduce switching losses and improve efficiency during light
load conditions.
When a light load condition occurs, the COMP pin voltage is
reduced by the voltage feedback loop to reduce the peak cur-
rent delivered by the controller. Referring to
PWM comparator input tracks the COMP pin voltage through
FIGURE 5. Waveform at SS and COMP Pin due to Brief Overload
Figure
FIGURE 6. Skip Cycle Control
6, the
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a 1.25V level shift circuit and a 3:1 resistor divider. As the
COMP pin voltage falls, the input to the PWM comparator falls
proportionately. When the PWM comparator input falls to
125mV, the Skip Cycle comparator detects the light load con-
dition and disables output pulses from the controller. The
controller continues to skip switching cycles until the power
supply output falls and the COMP pin voltage increases to
demand more output current. The number of cycles skipped
will depend on the load and the response time of the frequen-
cy compensation network. Eventually the COMP voltage will
increase when the voltage loop requires more current to sus-
tain the regulated output voltage. When the PWM comparator
input exceeds 130mV (5mV hysteresis), normal fixed fre-
quency switching resumes. Typical power supply designs will
produce a short burst of output pulses followed by a long skip
cycle interval. The average switching frequency in the Skip
Cycle mode can be a small fraction of the normal operating
frequency of the power supply.
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